Using a modified Doom engine, Heretic was one of the first first-person games to feature inventory manipulation and the ability to look up and down. It also introduced multiple gib objects spawned when a character suffered a death by extreme force or heat. Previously, the character would simply crumple into a heap. The game used randomized ambient sounds and noises, such as evil laughter, chains rattling, distantly ringing bells and water dripping in addition to the background music to further enhance the atmosphere. All music in the game was composed by Kevin Schilder. An indirect sequel, Hexen, was released a year later. Heretic II was released in 1998, which served as a direct sequel continuing the story.

Contents

Three brothers, known as the Serpent Riders, have used their powerful magic to possess seven kings of Parthoris into mindless puppets and corrupt their armies. The Sidhe elves resist the Serpent Riders' magic. The Serpent Riders thus declared the Sidhe as heretics and waged war against them. The Sidhe are forced to take a drastic measure to sever the natural power of the kings destroying them and their armies, but at the cost of weakening the elves' power, giving the Serpent riders an advantage to slay the elders. While the Sidhe retreat, one elf (revealed in the sequel to be named Corvus), sets off on a quest of vengeance against the weakest of the three Serpent Riders, D'Sparil. He travels through the "City of the Damned", the ruined capital of the Sidhe (its real name is revealed to be Silverspring in Heretic II), then past Hell's Maw and finally the Dome of D'Sparil.[3]

The player must first fight through the undead hordes infesting the site where the elders performed their ritual. At its end is the gateway to Hell's Maw, guarded by the Iron Liches. After defeating them, the player must seal the portal and so prevent further infestation, but after he enters the portal guarded by the Maulotaurs, he finds himself inside D'Sparil's dome. After the slaughter of D'sparil, Corvus ends up on a perilous journey with little hope to return home.

The gameplay of Heretic is heavily derived from Doom, with a level-based structure and an emphasis on finding the proper keys to progress. Many weapons are similar to those from Doom; the early weapons in particular are near-exact copies in functionality to those seen in Doom. Raven did however add a number of touches to Heretic that differentiated it from Doom, notably interactive environments, such as rushing water that pushes the player along, and items. In Heretic, the player can pick up many different items to use at their discretion. These items range from health potions to the "morph ovum", which transforms enemies into chickens. One of the most notable pickups that can be found is the "Tome of Power" which creates a much more powerful projectile from each weapon, some of which change the look of the projectile entirely. Heretic also features an improved version of the Doom engine, sporting the ability to look up and down within constraints, as well as fly. However, the rendering method for looking up and down merely uses a proportional pixel-shearing effect rather than any new rendering algorithm, which distorts the view considerably when looking at high-elevation angles.

As with Doom, Heretic contains different cheat codes that variously give the player invulnerability, all weapons, kill everything on a particular level. However, if the player uses the "all weapons" cheat code from Doom, a message appears warning the player against cheating and takes away all of his weapons, leaving him with only a quarterstaff. If the player uses the "god mode" cheat from Doom, the game will display a message saying "Trying to cheat, eh? Now you die!" and kills the player.

The original shareware release was the first game to come bundled with support for online multiplayer through the nascent DWANGO service.[4]

The original edition of Heretic was only available through shareware registration (i.e. mail order) and contains three episodes. A retail edition, distributed by GT Interactive, was titled Heretic: Shadow of the Serpent Riders and features two additional episodes: The Ossuary, which takes the player to the shattered remains of a world conquered by the Serpent Riders many centuries ago, and The Stagnant Demesne, where the player enters D'Sparil's birthplace. A free content patch was downloadable from Raven's website to update the original Heretic to match Shadow of the Serpent Riders.

In early 1999, the source code of the Heretic engine was published by Raven Software under a license that granted rights to non-commercial use, and was re-released under the GNU General Public License on September 4, 2008.[5] This resulted in ports to Linux, Amiga and other operating systems, and updates to the Heretic engine to utilize 3D acceleration. The shareware version of a console port for the Dreamcast was also released.

The Blasphemer project aims to create a free content package for the Heretic engine, with a theme of metal-inspired dark fantasy - in a similar way as Freedoom does with the Doom engine.